This is a request for continued support of projects designed to test the hypothesis that Excitatory Amino Acid (EAA) receptors mediate the neuronal plasticity that is manifest as opioid tolerance and dependence or as certain types of nociceptive behavior. These studies will use NMDA and non-NMDA receptor antagonists as well as antisense targeted to NMDA receptors in selected animal behavioral paradigms to demonstrate the role of EAA receptors in opioid tolerance and nociception. These antagonists include the open channel blockers, dextromethorphan, and the d and 1 isomers of methadone and newer antagonists acting at the glycine site, the NR2B subunit of NMDAR1 or the GluR5 subunit of non-NMDA receptors. Antisense oligos will be targeted to the NR1, NR2B and GluR5 subunits. To complement and extend these approaches, opioid tolerance, dependence and nociceptive behaviors will be measured in knockout mice engineered with an absent or mutated EAA receptor. A strategy is presented for the production of the first conditional knockout of the NR1 subunit that is confined to the dorsal horn of the spinal cord. Molecular and biochemical techniques including receptor binding, autoradiography, subunit specific ribonuclease protection assays and immunocytochemistry will be used to identify and localize to neuroanatomical areas, the changes in gene expression that are associated with these altered behaviors. Nociception, opioid tolerance and dependence are complex phenomena involving the alterations in integrated neuronal circuits (between systems events) as well as plasticity changes expressed in individual neurons. They appear to share a common locus, the EEA receptor system. An understanding of this system is important for the development of new and more selective drugs for the management of pain and opioid abuse.